Optical field traps, also called “optical tweezers”, “laser tweezers” or “optical traps” have been used for approximately two decades in the fields of biotechnology, medicine and molecular biology as well and other technical fields to position and manipulate micrometer-sized and submicrometer-sized particles (G. Weber et al. in Int. Rev. Cytol., Vol. 131, 1992, p. 1; S. M. Block in Noninvasive Techniques in Cell Biology, Wiley-Liss., New York 1990, p. 375). A. Ashkin has primarily started the development of laser tweezers (A. Ashkin in Phys. Rev. Lett., Vol. 24, 1970, p. 156). The principal of capturing particles by optically-induced forces is based on the fact that, in addition to radiation pressure that continuously pushes particles away from the light source, gradient forces arise which cause particles to move into a focus or be firmly held in it or move with it. Absorption and reflection of the particles must be low while the difference of the refractive index relative to the environmental solution should be as large as possible.
In recent years, laser tweezers have primarily gained acceptance because particles that are larger than the wavelength (Mie particles) as well as particles that are smaller than the wavelength (Rayleigh particles) can be trapped when the light beam is even and highly focussed. This includes biological objects such as cells, or organelles and other cell components as well as large molecules (such as DNA) and artificial microparticles (S. M. Block et al. in Nature, 1990, p. 348; E. M. Bonder et al. in J. Cell Biol., Vol. 11, 1990, p. 421).
W. Paul discovered electromagnetic field cages (W. Paul et al. in Forschungsberichte des Wirtschaftsministeriums Nordrhein-Westfalen, No. 415 and 450; W. Paul in Phys. Blätter, Vol. 46, 1990, p. 227). They are primarily used in elementary particle physics to trap and measure atomic particles under low gas pressures. In 1993, liquid-filled, three-dimensional microfield cages were presented for the first time using dielectrophoretic forces (T. Schnelle et al. in Biochim. Biophys. Acta, Vol. 1157, 1993, p. 127). If the conductivity of a suspension solution is higher than that of the average conductivity of the particle, the surface charges on the particle induced in the E field are arranged and polarized to make effective the forces acting on the particle (G. Fuhr et al. in Biochim. Biophys. Acta, Vol. 1201, 1994, p. 353). This principle can also be used to trap, position and move particles (T. Schnelle et al. in Biochim. Biophys. Acta, Vol. 1157, 1993, p. 127). Objects corresponding to the Mie and Rayleigh particles can also be held in these dielectric field cages. There have been numerous suggestions for combining both principles to increase the trapping force since the optical and electrical force fields scarcely interfere with each other (G. Fuhr et al. in Topics in Current Chemistry, Vol. 194, 1998, p. 84).